Electric rotating machines are required to have varying characteristics with different types of equipment in which they are used. For example, it is required that an electrical machine acts as a variable speed motor over a wide range as well as a high torque motor for low revolution speed operation when it is used, as a traction motor, in a hybrid electric vehicle (HEV) with an internal combustion engine or an electric vehicle (EV) as a driving source.
It is proposed for an electric machine with such characteristics to construct by adopting an interior permanent magnet (IPM) structure in which a plurality of pairs of permanent magnets are embedded in a rotor in a way that the magnets of each pair are located in a “V” shape configuration opening towards the rotor periphery because it is advantageous to use a structure that can effectively utilize reluctance torque together with magnetic torque (see e.g. patent literature 1).
Adopting the IPM structure enables an electric machine to make effective use of reluctance torque because q-axis magnetic path is kept by permanent magnets of each pair, embedded in a rotor, located in a “V” shape configuration. This increases the proportion of reluctance torque to magnetic torque and also saliency ratio (Ld/Lq), a ratio between inductance in d-axis and inductance in q-axis, resulting in increased tendency of space harmonics of the higher order to overlap flux waveform. The direct axis or d-axis is aligned with a direction of flux generated by magnetic poles and acts as a center axis between each pair of permanent magnets located in “V” shape, while the quadrature axis or q-axis is at an angle of 90 in electric degrees from the d-axis electrically and magnetically and acts as a center axis between the adjacent magnetic poles (i.e., the adjacent pairs of permanent magnets).
This causes high torque ripple, i.e., the difference between maximum and minimum torque during one revolution, in such electric rotating machine. The high torque ripple causes an increase in oscillation of the machine and electromagnetic noise. Especially, electromagnetic noise is desired to be reduced as much as possible because it gives an unpleasant sound to occupant(s) in a vehicle having, as an electric drive, the electric machine due to a relatively high frequency of the electromagnetic noise to that of noise generated by drive of an internal combustion engine.
On the other hand, oscillation becomes loss to cause a reduction in efficiency of performance of the electric machine although highly efficient performance is demanded to generate a desired driving force efficiently with less consumption of electricity.
Following not only restrictions of loading space, but also recent demands of improvement in energy conversion efficiency (mileage) in hybrid and electric cars, there is a growing demand of lightweight and miniaturization in electric rotating machines capable of providing high energy density output. Reducing torque ripple is effective to control judder, abnormal vibrations, and to provide smooth acceleration performance because, for example, there is a need to provide highly efficient drive over a usually used range for driving a car in street use.
It is very difficult to combine miniaturization as stand-alone units with improved efficiency, reduced electromagnetic noise and low torque ripple because, in electric rotating machines (motors), there are a tendency of increase in electromagnetic noise and a tendency of decrease in efficiency caused due to occurrence of torque ripple in accordance with an increase in output density per unit volume, but the demand of lightweight and miniaturization is growing.
In order to realize low electromagnetic noise and low torque ripple, it is proposed to axially divide a rotor to allow one of the adjacent pairs of permanent magnets to assume an angularly twisted positional relation with the other or give a skew angle (see, for example, patent literature 2).
The above-mentioned measure to give a skew angle in an electric rotating machine causes not only an increase in assembly cost and thus an increase in production cost, but also a difference at interfaces of the adjacent pairs of permanent magnets and a deterioration of the rate of magnetization at the interfaces, causing the permanent magnets to lower their magnetic flux density. As a result, the output torque to be produced by the electric rotating machine drops.
This is why various different ideas from the measure to give a skew angle are proposed to realize low electromagnetic noise and low torque ripple. They include an approach to modify an air gap between a rotor and a stator surrounding the rotor in such a way that an air-gap length at a position where every p-axis intersects the air-gap is greater than air-gap lengths at the other positions by, for example, modifying the shape of the rotor periphery in such away that the rotor periphery has a bulged shape at every magnetic pole like a “petal” shape (see, for example, patent literatures 1, 3 and 4).
In electric rotating machines described in patent literatures 1, 3 and 4, an inductance at every p-axis, which serves as a magnetic axis of one of magnetic poles created by permanent magnets on a rotor, increases because an air gap is wide, causing not only a drop in saliency ratio and a drop in torque, but also a decrease in machine efficiency.